Excretion of carbon dioxide through the lungs

Cells produce carbon dioxide as a waste product of respiration. The blood carries this carbon dioxide to the lungs, where it passes from the blood into the air in tiny air sacs called alveoli.

When that air is breathed out, the carbon dioxide leaves the body. This removal of a waste product from the body is called excretion.

Urinary excretion

The kidneys remove urea, excess water and excess ions from the blood. These substances form urine, which travels out of the body through the urinary excretion route.

The route of urine

Each kidney produces urine from substances removed from the blood.
Urine passes from each kidney through a tube called a ureter to the bladder.
The bladder stores the urine.
Urine leaves the bladder and passes out of the body through the urethra.

The ureters carry urine from the kidneys to the bladder; the urethra carries it from the bladder to the outside of the body.

Nephron filtration and urine formation

The kidney has an outer cortex and an inner medulla. Its filtering units are called nephrons, each of which is associated with blood vessels.

Filtration in the nephron

In the cortex, a cluster of blood vessels called the glomerulus filters substances from the blood into the nephron. Water, glucose, urea and ions pass into the nephron as part of the filtered fluid.

Reabsorption and urine formation

As the filtered fluid passes along the nephron, useful substances are taken back into the blood through the associated blood vessels. This is called reabsorption. Normally, all of the glucose, some of the ions and most of the water are reabsorbed.

The fluid that remains forms urine. It contains urea, excess water and excess ions. Filtration therefore does not mean that everything entering the nephron is lost from the body: much of the filtered material returns to the blood.

Assimilation of amino acids by the liver

After amino acids have been absorbed from digested food, the blood carries them to the liver. Liver cells join amino acids together to make proteins. This use of absorbed nutrients to make substances needed by the body is called assimilation.

The proteins made by the liver include proteins used within liver cells and proteins released into the blood. The absorbed amino acids are therefore incorporated into substances the body can use.

Deamination and urea formation

The body cannot store excess amino acids. In the liver, their nitrogen-containing part is removed. This process is called deamination. The liver uses the removed nitrogen-containing part to form urea.

Urea is toxic if it accumulates, so it must be removed from the body. It travels in the blood to the kidneys, which remove it from the blood. Urea then leaves the body in urine. The removal of a metabolic waste product such as urea is called excretion.